In studies of Japanese colonies of Culex pipiens group, the ommatidial number gave a reliable character with which to distinguish the autogenous form molestus from nonautogenous Culex pipiens pallens . In practice, the counting of the ommatidial number of the fourth or fifth row of the compound eye from the center on the dorsal side is most convenient. The actual figure is 7(♂) and 8(♀) in C. molestus against 8(♂) and 9(♀) in C. p. pallens . This could not be applied to our hybrid colonies, nor to an American colony of Culex pipiens .